How can I import my video properly?
From version 2.92 on, the scaling problem is resolved. So, I suggest that you upgrade your version. If you want to do it manually, you have to add a transform strip to the .3gp clip and set the scale yourself. You can find the original resolution of the clip in the properties panel under the section Source.
So, if your project resolution is 1920 x 1080 and your .3gp clip is 360 x 740, then your scale Y should be 1080 / 740 = 1.46. This will make the smartphone clip as tall as the project viewport. Of course, you should also scale the Y axis with the same amount (360 x 1.46 = 526 px) to avoid distortion (what you'll have automatically in the previous version), leaving a black bar at each side of the clip.